Recently, the organic EL element has been attracted considerable attention as a most suitable material for the flat panel display (FPD) having high brightness. Under this trend, a lot of the researches and developments have been carried out vigorously. The organic EL element has a structure wherein the luminescent layer is inserted between the two electrodes. The hole streaming in the anode positively charged recombines with the electron flowing in from the cathode negatively charged in the luminescent layer to become finally luminescent. The both materials of high molecular and low molecular can all be utilized to the production of the organic EL element. The both of them are proved to provide the organic EL element having high brightness.
The organic EL element is divided broadly into the two types. The one forms the luminescent layer by the use of the materials comprising the fluorescence dye to transporting the charges (See Journal of the Applied Physics, 65, 3610, 1989). The other utilizes the luminescence dye per se as the luminescent layer (See Japanese Journal of the Applied Physics, 27, L269, 1988).
The organic EL element utilizing the luminescence dye per se as the luminescent layer is further divided into the following three types. The first one is the three layers element wherein the luminescent layer is inserted between the hole transporting layer and the electron transporting layer, the second one is the two layers element wherein the hole transporting layer and the luminescent layer are laminated to the other one, and the third one is the two layers element wherein the electron transporting layer and the luminescent layer are laminated to the other one. So, the organic EL element has been known as exhibiting the improved luminescence efficiency in case that it consists of two or three layers.
In said organic EL element, the electron transporting layer comprises an electron transporting compound to function as transporting the electron from the cathode to the luminescent layer. Both of the hole injection layer and the hole transporting layer comprise the hole transporting substance to function as transporting the hole from the anode to the luminescent layer. When the hole injection layer is inserted between the anode and the luminescent layer, a number of increased holes can be flowed into the luminescent layer from the low electric field and the electrons streamed in from the cathode or the electron injection layer can be preserved restrictedly in the luminescent layer. Accordingly, the luminescent efficiency can be improved and thus the organic EL element with an excellent efficiency of the luminescence can be realized.
The various kinds of materials concentrated on triphenylamine derivatives used usually have been known widely as the materials used for such organic EL element. However, only very small number of materials are suitable for the practical use. N,N′-diphenyl-N,N′di(3-methylphenyl)-4,4′-diaminophenyl(TPD), for instance, has been informed (Applied Physics Letter, Vol. 57, No. 6, 531, 1990). However, this compound is thermally unstable and has a problem in respect to the life of element produced. Though lots of triphenylamine derivatives have been known (U.S. Pat. Nos. 5,047,686, 4,047,948 and 4,536,457, Japanese Patent Document No. 06-32307 and Japanese Laid Open Patent Application Nos. 05-234681, 5-239455, 8-87122 and 8-259940), the most of them are not satisfactory at the aspect of the feature.
Neither the star-burst amine derivatives disclosed either in Japanese Laid Open Patent Application No. 4-308688 or 6-1972, or in the literature of Advanced Material, Vol. 6, 577, 1994 satisfy neither the essential requirement for the organic EL element, i.e., the high luminous efficiency and the long life, nor satisfy it the respective compounds disclosed in Japanese Laid Open Patent Application Nos. 7-126226, 7-126615, 7-331238, 7-97355, 8-48656 and 8-100172 and the literature of Journal of the Chemical Society Chemical Communication, p2175, 1996.
The compound having thiophene-ring disclosed in the literature of Advanced Material Vol. 9, 720, 1997 has a defect to emit the long wavelength beam.
As set forth hereinabove, the materials used for the usual organic EL element has still been required the improved efficiency. Accordingly, an excellent material capable of improving the luminous efficiency has been desired.